An experimental and theoretical study of the thermal decomposition of chloride adducts of the nitro- and nitrate-containing explosives in an ion mobility spectrometer

Ion mobility spectrometry (IMS) is an analytical technique used to separate and detect trace amounts of gaseous ions based on their mobilities at atmospheric pressure under a linear electric field. IMS has been heavily employed in security and defense applications for detection of explosives and chemical warfare agents. Ion mobility spectrometers are operated in low field conditions, at which ions attain true thermal condition by reaching thermal equilibrium with supporting gas environment. Recently, these conditions have been used to determine the rate constants and thermo-chemical parameters for the dissociation of gas phase adduct ions of esters, ketones and organophosphates by IMS. These measurements were limited to positive polarity with the thermal dissociations of symmetrically proton bound dimers.;In this work, the same concept was extended to negative polarity with the dissociation of gas phase chloride adducts of explosive ions. Technological improvements such as sample pre-fractionation by a gas chromatograph, uniform heating of the whole drift tube, and mobility selection of ions by dual shutters for specific measurements were used to improve the accuracy of the results. The activation energies (Ea), rate constants (k) and standard enthalpy changes (DeltaH°) for the first order thermal dissociation reactions of the chloride adducts of nitro- and nitrate-containing explosives were determined. Ab initio calculations were used to validate the experimentally obtained thermo chemical terms. Reaction paths and products were also determined by theoretical calculations. It is revealed that the chloride adducts of NO3 containing explosives followed SN2 type displacement of NO3 by Cl. The chloride adducts of NO2 containing explosives followed a detachment reaction by simple loss of loosely held Cl.